The present invention relates to vascular prostheses, commonly referred to as xe2x80x9cstents,xe2x80x9d for maintaining the patency of a body vessel following a dilatation procedure, such as percutaneous transluminal coronary angioplasty. More specifically, the present invention relates to vascular prostheses formed of helical mesh coils, especially for use in exposed vessels and saphenous vein grafts.
A number of vascular prostheses are known for use in maintaining the patency of a body lumen following a dilatation procedure. Generally, in a procedure such as percutaneous transluminal angioplasty, a balloon catheter is inserted transluminally to the site of a stenosis within an artery, and the balloon is dilated to crack the plaque lining the artery. To prevent the dilated artery from restenosing, it has become common practice to insert a vascular prosthesis, commonly referred to as a stent, within the artery to maintain the artery at the dilated diameter. For example, the Palmaz stent, sold by Cordis Inc., Miami Lakes, Florida, the Gianturco-Rubin stent sold by Cook Cardiology, Inc., Indianapolis, Ind., and the Multi-Link stent sold by Advanced Cardiovascular Systems, Inc., Santa Clara, Calif., are commonly used following angioplasty in such a manner.
The foregoing stents, which are generally described in U.S. Pat. No. 4,739,762 to Palmaz, the U.S. Pat. No. 5,314,444 to Gianturco, and U.S. Pat. 5,421,955 to Lau et al., respectively, are representative of many of the balloon expandable stent designs currently for being offered for sale or under development. These stent designs employ a rigid member that is deployed by plastically deforming the member using a dilatation element, such as a balloon catheter.
A drawback of plastically deformable stents, however, is that such prostheses cannot be used in vessels that are close to the surface of the patient, and therefore are unprotected against crushing blows (such vessels referred to hereinafter as xe2x80x9cexposed vesselsxe2x80x9d). For example, if a plastically deformable stent is used in a carotid artery in the vicinity of the neck, even a minor neck injury could result in the stent collapsing in vivo, with potentially fatal consequences. Recent clinical trials of balloon expandable stents in exposed vessels have shown that up to 12% of the patients experience some collapse of the stent due to external forces.
Other stent designs which provide adequate. crush resistance are known, however, these previously known stent designs suffer from other drawbacks. For example, U.S. Pat. No. 5,443,500 to Sigwart and U.S. Pat. No. 5,344,426 to Lau et al. each describe a vascular prosthesis formed of a self-expanding coil sheet, while U.S. Pat. No. 5,423,885 to Williams describes a similar coiled sheet stent having a plurality of protrusions on its surface. A coiled sheet stent generally is rolled down to a small diameter, and then constrained within a delivery device at the small diameter. Once the stent is placed across a stenosis, a sheath of the delivery device is retracted, allowing the sheet to unroll. U.S. Pat. No. 5,556,413 to Lam describes a variation of a coiled sheet stent having a plurality of longitudinal slits so that the sheet forms helical coils when expanded. A drawback of coiled sheet prostheses, however, is that such prostheses generally are limited to use in vessels having relatively long lengths of uniform diameter, and which possess relatively low tortuosity.
U.S. Pat. No. 4,655,771 to Wallsten provides a woven wire tubular mesh member which is contracted to its delivery profile by elongating the stent. When the ends of the stent are released, the stent attains its expanded diameter by undergoing a considerable shortening of length. Drawbacks inherent in stents of this design include a limited range of diameters at which acceptable radial strength can be achieved, and relatively low longitudinal flexibility. In addition, the considerable shortening of the stent encountered during deployment can result in lack of precision during stent deployment.
U.S. Pat. No. 4,665,918 to Garza et al. describes a vascular prosthesis and delivery system for a self-expanding helical coil or coiled sheet. The helical coil is held in a constrained shape within an outer sheath of the delivery system, and is deployed by retracting the outer sheath. U.S. Pat. No. 5,147,370 to McNamara et al. describes a nitinol stent comprising a helical band having proximal and distal loops which is wound tightly onto a catheter and retained using a mandrel, so that the coil self-expands when released from restraint. U.S. Pat. No. 4,553,545 to Maass et al. describes similar helical coils formed from stainless steel and delivery systems therefore. All three of these patents suggest the use of a helical coil having a rectangular cross-section, while Maass further suggests that the coil may include punched openings to form a double helix structure.
While results of initial testing of helical band-type coil stents appeared promising, as described for example, in D. Maass et al., xe2x80x9cRadiological Follow-up of Transluminally Inserted Vascular Endoprosthesis: An Experimental Study Using Expanding Spiralsxe2x80x9d, Radiology 1984, Vol. 152, No. 3, pp. 659-663 (1984), concerns over the safety and efficacy of such designs have resulted in little effort to commercialize this technology. In particular, the tendency of the ends of the stent to project into the blood flow, as in the McNamara and Maass devices, is thought to promote thrombosis, while the large surface area contacted by the helical bands is thought to enhance restenosis.
Consequently, efforts to develop commercial systems using the coil-spring concept have concentrated on coiled springs made from nickel-titanium alloy wires, so as to minimize the contact area between the stent and the intima of the body vessel. For example, U.S. Pat. No. 5,246,445 to Yachia et al. (commercialized by Instent, Inc., Minneapolis, Minnesota), describes a helical wire coil that is drawn down onto a catheter for delivery by axially extending the catheter. The stent is deployed by releasing one end of the stent. U.S. Pat. No. 5,476,505 to Limon describes a similar helical wire coil stent.
Like the Wallsten device, the device described in the Yachia et al. patent experiences considerable longitudinal shortening during deployment. The device includes a further drawback that, as the device expands, the free end of the coil it believed to whip around the catheter at high speed. Because such behavior could dislodge pieces of plaque from the interior of the vessel wall, such stent designs appear unsuitable for use in the carotid arteries and in other vessels in which embolization presents a problem.
Previously known helical coil stent designs are thought to present a number of other drawbacks as well, such as having limited ranges of expanded diameters, the potential for tilting of coils and prolapse into gaps in a stenotic region, uneven expansion, migration, and thrombosis formation. For example, the devices described in the Maass et al. patent are expected to have only a limited range of expanded diameters due to the mechanical characteristics of stainless steel.
Likewise, the wire coils of the Yachia et al. device have been observed to expand unevenly, as well as to slip into cracks created in the plaque during the dilatation procedure, thereby creating nonuniform radial strength along the length of the stent and increasing the chance of restenosis. The smooth outer wall surface of the stents, as well as the narrowness of individual turns of the stent (both resulting from the use of coiled wire), also is thought to cause slipping and localized migration of turns, further reducing radial strength.
In addition, the potential for individual turns of the coil of the Yachia et al. device to project (either by tilting or overlapping neighboring turns) into the bloodstream, like the loops in the McNamara et al. device, enhances the risk for thrombosis. More generally, since the ends of a helical coil stent do not experience the same outward force as full turns of the coil, it is thought that the free ends may also project into the bloodstream, and hence serve as sites for thrombi formation.
In view of the foregoing, it would be desirable to provide a helical coil stent that overcomes the drawbacks of the previously known stents. In particular, it would be desirable to provide a helical coil stent that has uniform and reproducible radial strength over a range of expanded diameters.
It also would be desirable to provide a helical coil stent that conforms to a diameter of the body lumen, but which is crush resistant and will not experience localized slipping or migration of individual turns of the coil if loaded in compression after deployment.
It further would be desirable to provide a helical coil stent possessing a high degree of longitudinal flexibility so that it can be advanced through a tortuous body lumen, yet which has high radial strength over a range of expanded diameters and that experiences much less overall shortening during deployment then previously known helical coil stents.
It also would be desirable to provide a self-expanding helical coil stent that can be contracted to its delivery diameter with relatively few turns, so that the stent expands in a controlled manner without the high speed whipping action observed in previously known helical coil stents, thus reducing the risk of embolization.
In view of the foregoing, it is an object of the present invention to provide a helical coil stent that overcomes the drawbacks of the previously known stents relating to limited ranges of expanded diameters, tilting of coils and prolapse into gaps in a stenotic region, uneven expansion, migration, and thrombi formation.
It is therefore an object of the present invention to provide a helical coil stent that has uniform and reproducible radial strength over a range of expanded diameters.
It is another object of this invention to provide a helical coil stent that conforms to the diameter of the body lumen, but which is crush resistant and will not experience localized slipping or migration of individual turns of the coil if loaded in compression after deployment.
It is a further object of the invention to provide a helical coil stent possessing a high degree of longitudinal flexibility so that it can be advanced through a tortuous body lumen, yet which has high radial strength over a range of expanded diameters and that experiences much less overall shortening during deployment then previously known helical coil stents.
It is yet another object of this invention to provide a self-expanding helical coil stent that contracts to its delivery diameter with relatively few turns, so that the stent expands in a controlled manner without the high speed whipping action observed in previously known helical coil stents, thus reducing the risk of embolization.
These and other objects of the invention are accomplished by providing a stent comprising a self-expanding helical mesh coil having a substantially rectangular cross-section and a band width of at least about one-quarter of the maximum expanded circumference of the stent. The helical mesh has a multiplicity of openings forming a lattice which preferably provides about 60% open space or more. The openings serve to secure the stent within the body lumen, and serve to resist sliding or localized migration of the turns of the stent after deployment. In addition, the large band width of the helical mesh coil enables the stent to be reduced to its contracted diameter with very few turns, thereby providing controlled expansion without the whipping action associated with previously known stent designs.
Alternative embodiments of the helical mesh coil stent of the present invention may include specially designed free ends of the stent, which are treated to preferentially overlap neighboring turns of the coil when deployed, thereby ensuring that the free ends of the stent do not project into the body lumen. In addition, the helical mesh may include integrally formed barbs that, in addition to the multiplicity of openings, induce a ratcheting effect enabling the stent to resist localized compressive forces.
Methods and apparatus for deploying the helical mesh coil stent of the present invention are also provided. In accordance with these methods, the helical mesh coil stent is first deployed into a body lumen so that it expands when released to conform to the diameter of the body lumen. Delivery apparatus is provided that includes a retractable element for restraining either or both ends of the stent during deployment. The delivery apparatus also may serve to hold a first end of the stent in engagement with a wall of the body vessel during deployment, thereby enhancing accuracy of placement of the stent. A dilatation element (which may be tapered) is then disposed within the stent and expanded, thereby ensuring that the turns of the stent are uniformly expanded into contact with the intima of the body lumen. The large band width, in conjunction with the multiplicity of openings (and barbs, if present), serve to affix the stent in apposition to the body walls, without tilting or overlap.